Refractory vessel and lining therefor

ABSTRACT

A refractory vessel that can be used as a gasifier for black liquor includes a generally cylindrical metal shell having a dome. A refractory shell has a cylindrical portion spaced inwardly from the metal shell and a dome portion that is spaced inwardly from the metal dome. The refractory shell is sized to provide an expansion gap between the liner and the metal shell. A selectively crushable liner is positioned in the gap. The liner has a predetermined yield stress that will provide controlled resistance to expansion of the refractory shell.

FIELD OF THE INVENTION

The present invention relates to refractory vessels, and moreparticularly, to materials for lining refractory vessels.

BACKGROUND OF THE INVENTION

Black liquor is a by-product of the wood pulping process. Black liquoris a mixture of hydrocarbon, caustic, chlorine and other corrosivechemicals. It is normally completely combusted in a recovery boiler.Inorganic chemicals including sodium sulfate and sodium sulfide arerecovered for reuse in the pulping process. Heat produced by thecomplete combustion is converted to steam, which in turn is used toproduce process heat and/or electrical power. An alternative deviceproposed for recovering inorganic chemicals from black liquor is agasifier. In a gasifier, the black liquor is burned in a substoichiometric atmosphere to produce a combustible gas. Inorganic saltsare recovered in the process. The combustible gases can be used directlyto fuel a gas turbine, or combusted in a power boiler.

Low pressure gasification requires an insulated environment, which isobtained through a refractory lined vessel. Refractory vessels ofcurrent design for use as gasifiers employ a stainless steel jacket anda fused-cast alumina liner. The alumina liner normally has a first innerlayer of blocks comprising both alpha and beta alumina and a secondouter layer of blocks comprising beta alumina. A small expansionallowance is provided between the outer layer of beta alumina blocks andthe stainless steel jacket.

After vessels of this design are operated for a few months, it has beenfound that the refractory materials react with the soda in the liquorand expand to completely consume the normal expansion allowance providedbetween the refractory and the stainless steel jacket. At this point,the refractory layers begin to press against the inside of the stainlesssteel jacket. This situation causes early failure in the refractorymaterials themselves and plastic deformation of the stainless steeljacket. As a consequence, refractory linings of a conventional designhave been unsatisfactory for use in a black liquor gasifier.

SUMMARY OF THE INVENTION

As a result of a study of a prior gasifier, it has been found that thealumina refractory material has not only been subject to thermalexpansion, which was known in the prior art, but also is subject tochemical expansion. Chemical expansion is caused by sodium, present inthe black liquor, combining with the refractory material to producesodium aluminate. It has been found that sodium aluminate expands on theorder of 130% relative to the original alumina. This causes both greaterradial and vertical expansion in the refractory material as the vesselis used over time. This additional expansion of the refractory materialpresses outwardly on the interior of the stainless steel jacket andplaces it under stress. Chlorine and moisture present on the innersurface of the stainless steel pressure vessel, loaded in tension by therefractory expansion cause stress corrosion cracking of the stainlesssteel vessel, subjecting it to early failure. This internal stresscaused by the thermal and chemical expansion of the refractory must becontrolled to an acceptable level. Also, for the purpose of pressurevessel design as per ASME code, it is necessary to clearly andexplicitly define the “secondary stress” due to the expansion of therefractory lining. Secondary stresses are all stresses not due to theinternal gas pressure.

The present invention therefore provides a refractory liner for a vesselwhich accommodates the expansion of the refractory lining and provides aknown secondary stress on the pressure vessel. The vessel has acylindrical metal shell preferably having a hemispherical dome. Arefractory shell has a cylindrical portion spaced inwardly from themetal shell and preferably a hemispherical dome portion spaced inwardlyfrom the hemispherical dome of the metal shell. The refractory shell issized to leave a uniform expansion gap between the liner and the shellin the cylindrical section.

In a preferred embodiment, the center of curvature of the hemisphericaldome comprised of the refractory is at a lower elevation than the centerof curvature of the hemispherical dome comprised of the metal shell.This provides an expansion gap which increases in thickness as the domecurves upwardly and inwardly. This crescent shaped gap in the domeallows for radial expansion of the refractory dome as well as axialexpansion of the cylindrical section. The entire refractory dome risesin the vertical direction as the cylindrical section expands. Aselectively crushable liner is positioned in the gap. The liner has apredetermined yield and crushing stress that will provide controlledresistance to expansion of the refractory shell. The resistance providedis significantly less than the yield strength of the metal shell whileproviding sufficient resistance to expansion of the refractory shell toallow controlled growth of the shell. Since the mechanicalcharacteristics of the crushable liner are known, the internal secondarystress on the steel shell can be accurately described.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is an isometric view of a refractory vessel constructed inaccordance with the present invention with a vertical pie-shaped segmentremoved to expose the refractory lining and crushable liner; and

FIG. 2 is an enlarged view of a portion of the vertical wall and dome ofthe refractory vessel shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, the refractory vessel 10 has an outer metalshell 12. The outer metal shell is preferably comprised of carbon steelbut can be composed of any other suitable material with adequatestrength and corrosion resistance. The upper portion of the metal shellcomprises a dome 14 that terminates in an upper opening 15. The bottomportion of the metal shell 12 merges into a support cone 16 having acentral bottom opening 17. A refractory liner 20 has a cylindricalportion 22 positioned radially inward from the shell 12 and also has adome portion 24 and a bottom cone portion 26. A cylindrical expansiongap 27 is provided between the metal shell 12 and the cylindricalportion 22 of the refractory liner 20. The dome portion of therefractory liner is positioned inwardly and below the dome 14 of themetal shell.

Referring to FIGS. 1 and 2, in a preferred embodiment, the upper portion24 of the refractory liner 20 is hemispherical in shape. The center ofcurvature of the hemispherical dome 24 of the refractory liner 20 is ata lower elevation than the center of curvature of the hemispherical domeportion 14 of the metal shell 12. This provides an expansion gap 28which increases in thickness as the two hemispherical portions 14 and 16extend upwardly and inwardly toward the opening 15. Expansion gap 28connects with the cylindrical expansion gap 27. A selectively crushablelayer 70 is positioned between the refractory liner 30 and the outershell 12. The crushable layer 70 is described in more detail below.

The refractory liner 20 has an inner layer of blocks 34 and an outerlayer of blocks 30. The outer layer of blocks 30 are stacked on eachother to form an outer refractory shell and the inner layer of blocksare stacked on each other to form an inner refractory shell. The blocksin the inner layer are preferably comprised of alumina and mostpreferably of alpha and beta alumina. The blocks in the outer layer arepositioned in intimate contact with the outside of the inner layer ofblocks and are preferably composed of beta alumina. However, otherrefractory materials with suitable strength and resistance to chemicalattack could be used. The crushable layer 70 is positioned between theouter surface of the outer layer of blocks 30 and the interior surfaceof the metal shell 12. The width of the gaps 27 and 28 are adjustedbased on the measured or expected expansion of the refractory material.

Referring to FIGS. 1 and 2, the hemispherical dome 24 of the refractoryliner is formed by a plurality of rings of blocks 40, 42, 44, 46, 48,50, 52, 54 and 56 positioned on the blocks 30 and 34 forming the innerand outer cylindrical shell. Blocks 40 form a first horizontal ringcomprising the base of the hemispherical refractory dome. Successivelayers of blocks 42, 44 and 46 are formed into rings of lesser diameterto form the bottom portion of the inwardly and upwardly sloping dome.Each of the successive layers have flat upper and lower surfaces thatare appropriately angled relative to each other to form the dome shape.The next successive layer of blocks 48 also has a lesser diameter thanthe previous layer of blocks 46. Blocks 48 have a flat bottom surfaceformed to contact the flat top of the blocks 46 of the previous layer.However, the upper surface of the layer blocks 48 has a downwardlyextending circular keyway 48 a positioned in upper surface of the blocks48 adjacent their outer edges. The next successive layer of blocks 50has a lesser diameter than the layer of blocks 48 and has a downwardlyextending circular key 50 b positioned adjacent the lower outer edges ofthe blocks 50. Downwardly extending key 50 b extends into and mates withthe keyway 48 a in blocks 48. Similarly, the next set of blocks 52 alsoforms a ring of lesser diameter than that of the layer formed by blocks50. Blocks 52 have a downwardly extending circular key 52 b thatsimilarly engages a corresponding keyway 50 a in the preceding layerformed by blocks 50. The next successive layer of blocks 54 have acircular key 54 b that similarly mates with a circular keyway 52 a inblocks 52. The final layer of blocks 56 is positioned upwardly andinwardly from the layer of blocks 54. Blocks 54 have a horizontal bevel54 a on their upper surface. Blocks 56 have an outwardly extendingflange portion 56 b that overlies the bevel 54 a. Thus, each successivelayer of blocks from the layer formed by blocks 48 through the layerformed by blocks 56 are keyed into the next preceding layer andrestrained from falling downwardly or inwardly as differential expansionof the refractory materials occur.

A second hemispherical layer of blocks 60 may be positioned outwardlyfrom blocks 40 to 56. These blocks are conventional in design that haveslightly beveled edges to mate to form the hemispherical curve.

Based on studies of the prior failure in refractory vessels used forgasifiers, it has been found that the refractory liner 20 must beallowed to expand outwardly and upwardly a certain distance, otherwisethe inner surface of the refractory will fail due to excess spalling andcracking caused by the vertical and radial expansion. On the other hand,the refractory liner cannot be allowed to expand too quickly, or thegrowth rate will exceed the structural limitations of the liner and willultimately lead to structural failure. It has been postulated for thealumina-type refractory materials that if a predetermined resistance toexpansion is provided, the thermal expansion rate can be inhibited in acontrolled manner while still allowing sufficient expansion to eliminateexcess spalling from the inner surface of the refractory. This internalcompression stress (ICS), that is resistance against expansion, may bedefined by the formula (for the cylindrical section)${ICS} = \frac{2 \times {yield}\quad {stress} \times {shell}\quad {thickness}}{{shell}\quad {diameter}}$

wherein the yield stress is yield stress of the a stainless steel metalshell used in a prior art, thickness is the thickness of the metal shellused in a prior art, and D is the diameter of the metal shell used in aprior art. For a typical refractory vessel used in a gasifier, this willresult in an internal compression stress of about 2 MPa. This internalcompression stress can be provided by a crushable liner 40 that has ayield stress of about 2 MPa at 65% strain, defined as

(initial thickness−final thickness)/initial thickness.

When that yield stress is exceeded, the crushable liner willirreversibly compress but will still resist radial expansion of therefractory liner 30 with a force equivalent to the internal compressionstress.

The yield stress of the crushable layer may be varied, depending uponthe composition of the refractory material, the composition of the outershell, as well as the dimensions of the vessel. In practice the yieldstress is maintained in the range of from 0.5 to 4.0 MPa, morepreferably from 1.0 to 3.0 MPa, and most preferably from 1.5 to 2.5 MPa.

One material that will function in this environment is foam materialavailable under the trademark Fecralloy™FeCrAlY, which is aniron-chromium-aluminum-yttrium alloy. This material is an alloy withnominal composition by weight %, respectively, of _(—)72.8_% iron,_(—)22_% chromium, _(—)5_% aluminum, and _(—)0.1_% yttrium and 0.1%zirconium. This metal foam is produced commercially by Porvair Fuel CellTechnology, 700 Shepherd Streel, Hendersonville, N.C. It has furtherbeen found that the yield stress of this metal foam, that is thecompression stress at which the material will irreversibly begin tocompress, can be varied depending upon the density of the foam. Forexample, a foam having a density on the order of 3-4% relative densitywill have a yield strength of about 1 MPa. A material having a relativedensity of about 4.5-6% will have a yield strength of approximately 2MPa, while a material having a relative density greater than about 6%will have a yield strength of about 3 MPa or greater. Thus, a materialhaving a yield strength of about 2 MPa has been found to be mostdesirable for use as a crushable liner 40 for refractory vessels used inthe gasifier environment. Other metal foams composed of stainless steel,carbon steel, other suitable metals and metal alloys that have theforegoing properties can also be used.

As the alumina refractory material is exposed to process conditions,over time the typical refractory liner will expand about 1 inch in theradial direction per year. It is therefore desirable to provide acrushable liner 40 that has an original thickness which allows acompression of 1 inch while providing a yield strength of less than orequal to 2 MPa.

Another desired characteristic of the crushable liner 40 is that it mustbe sufficiently conductive so as to maintain the temperature of thecrushable liner under approximately 600° C. It has been postulated thatbelow this temperature, certain species produced in the gasifier willcondense to a solid. If such condensation is allowed to occur in thefoam lining, it will fill with solid over time and lose itscrushability, therefore becoming ineffective to selectively resistexpansion of the refractory liner. It has been found that the compositemetal foams just described have an adequate thermal conductivity on theorder of 0.5 W/mK to maintain the outer surface of the brick at atemperature under 600° C. Thus, any gaseous species will condense in therefractory itself, as opposed to the metal foam, thus allowing the metalfoam to retain its selective crushability.

The metal from which the shell 12 is made can be carbon steel, stainlesssteel, or any other suitable alloy. One of ordinary skill will be ableto choose other crushable materials that will exhibit the controlledcrushability characteristics of the metal foam after understanding therequirements for controlled crushability and substantially constantresistance to expansion over the limited distance between the refractorymaterial and the outer shell of the vessel, as outlined above.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A refractory liner for avessel comprising: a generally cylindrical metal shell having a dome;and a refractory liner having a cylindrical portion spaced inwardly fromsaid shell and a dome portion spaced inwardly from the dome of saidshell, said refractory liner being sized to provide an expansion gapbetween said liner and said shell, and a selectively crushable materialpositioned in said gap, said material having a predetermined yieldstress that will provide controlled resistance to expansion of saidrefractory shell, wherein said crushable material comprises a crushablemetal foam.
 2. The apparatus of claim 1, wherein the thermalconductivity of said foam is on the order of 0.5 W/mK±10%.